A cellulose ester film has been used as a support of a silver halide photographic light sensitive material. Recently, the cellulose ester film has been also used as a protective film of a polarizing plate used in a liquid crystal display. However, a property, which has not been required in the silver halide photographic light sensitive material, is required in the protective film of the polarizing plate. A plasticizer is contained in a cellulose ester film used for a protective film of a polarizing plate as in a support of a silver halide photographic light sensitive material so that the cellulose ester film has flexibility or water resistance.
Recently, development has been made regarding decrease in thickness and weight of an information device with a liquid crystal display such as a note board type personal computer, a car navigation system, a cell phone or a game device. In parallel with this development, a protective film of a polarizing plate has been eagerly sought to be increasingly thinner. Accordingly, the protective film of the polarizing plate is required to be far thinner than the cellulose ester film support of the silver halide photographic light sensitive material. However, a decrease in thickness of the film increases moisture vapor permeability, which lowers performance of a liquid crystal display employing the film at high humidity, particularly at high humidity and high temperature.
Further, a simple decrease in thickness of the cellulose ester film increases moisture vapor permeability, minimizes water resistance, and occasionally deteriorates a polarizing film or an adhesive used to adhere the polarizing film to a cellulose ester film (as a protective film of a polarizing plate). In order to compensate for the decrease in thickness, an addition amount of a plasticizer in the film is ordinarily increased. However, it has been found that a simple increase of the plasticizer addition amount causes precipitations on the film surface, and produces new problems in addition to those occurring in the silver halide photographic light sensitive materials. There is a problem that the increased addition amount of plasticizers lowers a glass transition point (hereinafter referred to also as Tg) of the cellulose ester film to soften the cellulose ester film, resulting in a lowering of dimensional stability of the film, (for example, increase of rate of shrinkage, coefficient of hygroscopic expansion or coefficient of thermal expansion).
After a cellulose ester dope containing a plasticizer is cast on a moving endless metal support to form a film web employing a solution casting film forming method, the web is dried. During drying, the plasticizer moves in the thickness direction of the web, or is evaporated and condensed on the walls of the dryer to form plasticizer droplets. The droplets result in contamination of the rollers and web.
The weight of the cellulose ester film decreases at high humidity and high temperature due to volatilization or precipitation of additives such as plasticizers. A property that additives such as plasticizers are retained in the cellulose ester film is referred to as a retention property. Conventional cellulose ester films are low in the retention property, which lowers performance of a liquid crystal display employing the cellulose ester film.
In order to solve the above problems, there are proposed techniques that polymers such as polyesters, polyesterethers, or polyurethanethers are added as polymer plasticizers or together with low molecular weight plasticizers, to the cellulose ester film, which are disclosed for example, in Japanese Patent Publication Nos. 47-760 and 43-16305, Japanese Patent O.P.I. Publication Nos. 5-197073, and U.S. Pat. Nos. 3,054,673 and 3,277,031. There is a proposal in U.S. Pat. No. 3,277,032 that methyl acrylate is polymerized in the cellulose ester, and the resulting polymethyl acrylate is blended with the cellulose ester. However, a problem has been found that the addition of the polymer plasticizers causes phase separation in the cellulose ester dope, in the web during drying in cellulose ester film manufacture or in the cellulose ester film itself, which results in lowered film transparency, increased moisture transmittance, non-uniform film shrinkage, or deterioration of rate of shrinkage.